Deposition of metal on nonmetal filament



DEPosrrIoN or METAL N NoNME'rAL FILAMENT Theodore T. Magel, Cambridge,Mass., assigner to the United States of America as represented by theUnited States Atomic Energy Commission Application .Fully 23, 1947,Serial No. 762,839 Claims. (Cl. 7584) melting point of the particularmetal, whereby the halide O' decomposes and the metal is deposited onthe filament, the system being static and being maintained under lowpressure. This prior system has not been found satisfactory forproducing pure massive metals that have an atomic number of 90 orgreater, or which are highly reactive at the higher temperatures,especially above their melting points.

Hence, broadly an object of the present invention is to produce a metal,particularly uranium, of extremely high purity in massive form bydecomposition of a volatile compound of the metal at a temperature abovethe melting point of the metal. Y Another object is to produce metal ofextremely high purity from a source or mass of the impure metal by anovel continuous-flow hot-filament method in which a heated surfaceoperates at a temperature above the melting point of the particularmetal being prepared and preferably is substantially nonreactive withthe metal at temperatures above the melting point of the latter.

Another object is to produce metals of extremely high purity from animpure source of the metal by a novel continuous-tlow hot-filamentmethod which may be accurately and eiiiciently controlled.

Other objects and advantages will become apparent from the followingdescription wherein reference is made to the accompanying drawings inwhich:

Fig. 1 is a diagrammatic side elevation, parts being in section, of anapparatus suitable for practicing the present invention; and

Fig. 2 is a cross section of a modification of the reaction receptacleof the apparatus lillustrated in Fig. 1.

For purpose of illustration, a continuous-flow method purity from animpure mass of uranium metal is disclosed, the broader aspects of theinvention and its application to other metals being apparent from theillustrative example and the appended claims.

Broadly speaking, the method comprises volatilizing a metal halide andexposing the halide vapor to a source `of heat which is maintained at atemperature above the melting point of the particular metal beingprepared so` that the metal halide decomposes and the metal isprecipitated in molten form and `collects in massive form.

In the` application Vto uranium metal, a halogen such as iodine vapor,preferably in the presence of a sweeping gas, such as hydrogen, ispassed continuously over.

an impure mass of uranium metal at very low pressure States Patent O forthe preparation of uranium metal of extremely high and in the presenceof suicient heat to cause the production of a uranium iodide vapor whichimmediately is passed over and is decomposed on a refractory, surface,such as a thoria-coated tungsten wire or special carbon filament whichis heated to a temperature above the melting point of uranium and issubstantially nonreactive with respect to the uranium at saidtemperature. The pure uranium metal is precipitated on the refractorysurface and continuously drips olf, falling into a suitable refractorycontainer which is located so that the dripping uranium metal coolssuiciently during its fall to be substantially unreactive with therefractory container when it is received therein. The waste gases areswept out of the system so that equilibruim conditions 'are preventedand poisons are removed, thereby making the operation continuous.

It is desirable to maintain the pressure in the system very 10W so as todrive the reaction in the direction of decomposition and precipitationof the metal. By supplying controlled amounts of iodine vapor andhydrogen and maintaining the system at very low pressures, the iodinevapor, which would tend to reverse the reaction if excessive in amount,can be maintained more nearly at the exact amount required. Of thesevery low pressures, which are usually less than 1 mm. of mercury, apressure in the system of about 0.10 mm. to as low as 8X10-5 mm. ofmercury has proven most satisfactory.

Referring to Fig. 1 an apparatus which has been found suitable forpracticing the above method as a iiow or static method is illustrated;it `comprises a furnace 1 in which is installed a reaction chamber orreceptacle 2, preferably of quartz, near one end of which is an exhausttube 3 -which leads to a vacuum pump of the mercury diffusion type (notshown). Extending into the opposite end of the reaction chamber 2 is aninlet tube 4 having a restricted orifice 5 for producing a jet, and anextension, or support 6, which extends therefrom and is positioned tosupport a piece or mass of impure uranium metal in alignment with theorifice 5. A filament 7, preferably a wire, is adapted to conduct, andbe heated by, electricity, and it is connected to lead wires whichextend through the end of chamber 2 that is opposite inlet duct 4. Thefilament 7 is positioned in alignment with the orifice 5. To facilitatethe removal of the filament, its lead wires are mounted in a groundglass stopper capable of a sealed lit with the end of the chamber 2.

The chamber 2 has a dependent well portion 8 arranged to accommodate arefractory receiving container 9 in a Aposition directly beneath thefilament 7, the spacing of the receiving containerv 9 beneath thefilament 7 being sufficient to permit molten uranium metal dripping fromthe filament to cool sufficiently to be substantially nonreactive withthe receiving container 9.

For supplying controlled amounts of halogen and sweeping gas into thechamber 2, the inlet tube 4 vis connected by one branch to a receptacle10 from which a halogen, such as iodine vapor, may be introducedunderthe control of a stopcock 11, and by a second branch to a receptacle 12.In preparing a purer uranium, the sweeping Vgas introduced into theinlet duct 4 with the halogen is hydrogen which is preferably in a verypure and dry condition and the flow rate of which is controllable. Forobtaining a controlled ow of pure dry hydrogen, said second branch ofthe tube 4 is connected to theoutlet of the receptacle 12 in which is apalladium thimble 13. The receptacle 12 is located Within a furnace 14and its inlet is connected to a drying receptacle 15 into which hydrogenmay be introduced from a suitable source (not shown), a purifying trap16, preferably a lliquid air trap, beinginterposed between the dryingreceptacle 15 and the receptacle 12, if desired. The receptacle 15contains an agent for drying the sweeping purpose.

and cooling of the palladium thimble in the receptacle 12,`additionalcontrol being provided by a suitable stopcock interposed between thereceptacle v12 and the recep'tacle it).

The halogen shown illustratively is iodine vapor and is supplied from amass of iodine `crystals in the receptacle 10, the rate of flow `ofiodine vapor therefrom being controlled by the temperature and shape ofthe receptacle l10.

The mixture of iodine vapor and hydrogen incontrolled amounts is 'passedthrough the orifice V5 and impinges as a jet on fthe mass Ni yof impureuranium metal on the supporte. -At'thepoint'of impingement of the jetonto the metal the 'receptacle 2 is maintained at a temperature at whichura'riiur'n halide vapor is 'formed as a result of the reaction of thehalogen with the uranium metal. A temperature `of ifrom 450-550 C. issatisfactory for this The uranium iodide vapor thus formed, along withany excess hydrogen and iodine vapor, contacts the filament 7 which isheated'to a temperature above the melting point of the uranium, forexample, to from l350 C. to 1500" C. Where it is decomposed by the heat.Uranium of a very high degree of purity forms in small globules on thefilament 7 as a result of this decomposition and 'these coalesce intodroplets of sufficient size to drip from the filament 7 into thereceiving container 9 which is positioned therebeneath so that when thedroplets reach the coilta'iiier 9,' they have become cooled below altemperature a't which the metal reacts appreciably with the container9. Waste gases and decomposition products are Withdrawn from the systemthrough Vthe exhaust tube 3 vby the high-capacity vacuum pumps.

Referring to Fig. 2, areactio'n chamber 17 is illustrated, this chamberbeing in all respects the same as `chamber 2 except that it is madeofPyrex glass instead of quartz. The use of Pyrex glass instead of the-more expensive quartz is madevpossible by the introduction of ametallic radiation' shield 158, such as a tantalum shield, whichsurrounds the filament in spaced relation thereto and to the walls ofthe receptacle 17. The shield is provided with apertures 19 so arrangedto permit viewing lof the filament and to per'r'nit the dripping of themetal from the filament into a receiving container 9. The function ofthe tantalum shield l18 is to prevent aging and deterioration oftheYglass which wouldbe occasioned by a direct exposure thereoftoradiation.

In operation, to provide a continuous flow, the apparatus, which issealed between the source of hydrogen and the `exhaust tube 3, is firstflushed with hydrogen and then evacuated. VThese steps are repeatedalternately until the system is thoroughly cleansed and until thefurnace and filament vhave reached the desired temperatures; asatisfactory vacuum is finally established in the chamber 2. The iodineis brought to the desired temperature. The furnace is heated to producea temperature of S15-520 C. in the reaction chamber 2 at the location ofthe metal M. The filament temperature is raised to 1350-1500" C. and inthe case of uranium to 1450" C. A few 'experiments will be necessary fordetermining the temperatures in the case of metals other than uranium.

For contact with uranium, because of its high degree of reactivity latelevated temperatures, a refractory material has to beused that isnon-reactive with the uranium above the melting-point of the latter. Afilament found satisfactory for use with uranium is a tungsten Wirecoated with thoria. A Nernst type filament is particularly good.

The present novel hot-filamentcontinuous-flow method of producing puremetals, therefore, vhas certain definite advantages among which are thatit permits operation above 'the melting point of the metal beingproduced, employs to an advantage a refractory-coated filament, usesdirectly the impure metal as a source instead of iodides or othervolatile metal compounds that had to be prepared first, permits veryaccurate control Without danger of contamination, and produces a metalof very high purity.

During operation at least one of the constitutents of thehigh-temperature decomposition or reduction is removed continuously bythe sweeping gas and the vacuum pump so that equilibrium is neverestablished and high purity metal is continuously produced at arelaitvely constant rate.

A number of factors can be varied for effecting control of the operationand for rendering it applicable to metals other than the uraniumillustratively described.

Among these factors are the temperature of the metal, filamenttemperature, pressures, rate of halogen flow, rate of sweeping gas orhydrogen flow, and also the surface characteristics of the impure metalsource.

The temperature in the vaporizing portion of lthe reaction chambershould be maintained high enough to cause rapid and complete reaction ofthe iodine vapor and metal to produce metal iodide vapor, but at 'thesame time the temperature should be sufficiently low so as not to causeapreciable decomposition of the iodide. ln the case of iodine Vapor-anduranium, the preferred temperature appears to lie between 450 C. and 550C. for the vaporizing portion of the chamber, though the metaltemperature may be slightly higher.

The temperature in the decomposition portion, in this example thefilament, should be high enough to reverse the initial reaction. 'Ehehigher the filament temperature, the greater the tendency of the voltilemetal iodide to decompose. For uranium, a filament temperature of l350C. appears to be about the minimum temperature for an appreciablereaction rate and operation has been successful at as high a temperatureas 1500 C.; about 1450 C. is preferred. The metal iodide decomposes onthe heated surface of the coated filament and the purified uranium metalformed deposits thereon. The deposited uranium metal, in turn, providesa surface on which additional uranium iodide is decomposed. Since thesurface area of the metal on the filament increases until the metalglobules formed drip olf, the effective decomposition surface variesperiodically within certain limits but maintains itself at a ratherconstant average for any substantial period of operation.

The rate of iodine flow is proportioned to the state of subdivision ofthe impure uranium source. Assuming that the surface area of the uraniumsource is lconstant, the maximum iodine flow is limited ronly by theamount of iodine vapor that can react with the uranium. Any excess ofiodine above this maximum tends to reverse the reaction at or beyond thefilament by combining With metal already precipitated on the filament.Obviously, the iodine flow can be adjusted, and it is increased anddecreased in a relationship directly proportional to an increase anddecrease, respectively, of the surface area of the metal source.

.As mentioned, the rate of fiow of iodine vapor is readily controlled bythe temperature at which the iodine is maintained, the shape of thereceptacle y1t) in which the iodine crystals are contained, the degreeof low pressure in the system, and the amount of other gas,'suchvas'hydrogen, which is introduced.

The fiow of hydrogen is controlled by the temperature of the palladiumthimble 13 through which the hydrogen is passed before it enters tube 4and bywhich it is purified. The sweeping gas used, in one embodiment ofthe invention, should be one which also reacts with the halogen freed bydecomposition of the halide to 'prevent the freed halogen fromrecombining with the precipitated metal. In the case vof uranium andiodine vapor hydrogen appears to operate in this-manner. Furthermore thehydrogen may'catalyze the formation of uranium iodide at lowtemperatures by forming uranium hydride, or by forming HI which in turnreacts with the uranium. Again, the hydrogen vtends to drive the hightemperature reaction in the direction of metal production by combiningwith excess iodine vapor originally introduced, or formed as a result ofdecomposition of uranium iodide, and thus causes its immediate removal.Another possibility is an indirect catalyticlelect such as the removalof poisons on the surfaces involved. For example, the hydrogen maymaintain the surface ilm of the impure uranium metal more porousorsusceptible to the action of the iodine vapor. Another effect whichappears to result from the introduction of hydrogen along withcontrolled amounts of iodine vapor into a low pressure system is thatthe amount of iodine vapor which would tend to reverse the reaction atthe filament if in excessive amounts can be more nearly maintained atthe exact amount required.

Hydrogen is not absolutely necessary for the production of some uranium4metal by this invention, but it very greatly increases the yield. `Itshall be noted that the present invention shall not be limited4 toA acontinuousflow system inasmuch as the method ofpreparing these metals,such as uranium, may be successfully accomplished by batch operationsperformed by closing oil the reaction system. This method of preparationis satisfactory, although the metal yields are extremely low incomparison to the ow method.

The surface of the lament should bea refractory which is substantiallynonreactivewith the `particular metal being prepared at the temperaturesabove the melting point of the metal, preferably a refractory inorganiccompound, or a nonmetallic filament, such as a filament of the Nernsttype, should be used. Thoria, lanthanaand urania-coated filaments oftungstenhave proven satisfactory in the case of uranium. Uranium alloyswith tungsten, tantalum and other metals and causes laments to burn outif the latter are exposed to the uranium above its melting point for `anappreciable interval. It is desirable that the filament be of as high adegree of purity as practicable.

For other metals, such as plutonium, thorium, cerium,

titanium, it is advantageous to use other types of laments, such asNernst-type filaments, which are refractory, with 85 percent thoriaand15percent'yttria, and filaments with no metal at all. The filamenttemperature which will give appreciable yields of the metal desiredwithout reacting with the metal or alloys thereof determines the typepreferred in a particular case;` it should be borne in mind that thelower the operating temperatures required, the less is the chance foralloy formation.

so that uncoated metal filaments are useful for some purposes. Thereceptacle 9 may be of thoria, cerous sulfide, or even platinum.

In a preferred operation in thecase of uranium, iodine vapor andhydrogen, the temperature in the reaction chamber is maintained at 515C. to 520 C., the filament is maintained at a temperature of about 1450C., the pressure'in the system, in mm. of mercury, is maintained atabout 8X105 when tirst evacuated, `the introduced iodine vapor is heldat about 0.9 mm., the hydrogen pressure isheld from about 3 l03 to lmm., and the ratio of hydrogen to iodine vapor is about 5 to l.

While the above invention has been described illustratively as appliedto uranium, it is applicable to other metals, including tungsten,vanadium, titanium, thorium, hafnium, zirconium, plutonium, neptuniumand the rare earths.

In addition to the thoria coating on the tungsten wire, other refractorysurfaces may be provided. These include lanthana for tungsten, tantalumnitride for tantalum, and also a properly machined graphite rod isusable; all of these are unreactive with metals at temperatures as highas 1650 C.

. K 6 v I For preparing a thoria-coated tungsten lilament a small pieceof tungsten wire is spot-welded across the gap between two ordinarypieces of tungsten and then is covered with a thoria-containing slurryeither by dipping or by means of a dropper. The slurry coating is driedin air and then the wire is placed in a chamber where it is subjected toa vacuum While the temperature is slowly brought up to l650 C.; it ismaintained at that temperature for a while, the entire procedurerequiring from one to'two hours. The slurry coating preferably is madeas follows: 25 drops of a saturated thorium nitrate solution in waterare added to 5 cc. of a very thick thoria paste and suicient water isadded to give the proper consistency. In some instances, where ytheexclusion of water is desirable, anhydrous materials, e. g. amyl orother alcohol, are used as solvents instead of water.

The lanthana-coated tungsten filament is prepared in the same manner,except for the ingredients. Lanthana itself is extremely hard todehydrate lbut a slurry of anhydrous lanthana in ethyl alcohol providesa coating which is suitable after the baking procedure.

Another filament is one of tantalum coated with tantalum nitrideobtained by heating a tantalum wire very rapidly to an extremely hightemperature in an atmosphere of nitrogen. None of these filaments thuscoated reacts with metals deposited on them even at temperatures as highas l650 C., but the base metals of the filaments would react.

Ordinary carbon filaments have been found unsatisfactory. However, afilament prepared by machining an ordinary preformed graphite rod to avery small crosssection with a portion having a cross-section of lessthan normal was found satisfactory. The lament is then tapered toward`the smallest-diameter portion from a short distance on either sidethereof. Such vallilament has a section of gradually decreasingcross-section toward the intersection of the tapers and thus of varyingresistance. The heat at any given point along the tapered portion of thefilament depends directly upon the crosssection.

It shall be noted that other refractories can be used, such as thoriumnitride, zirconium nitride, tantalum nitride, uranium nitride, andtitanium nitride. These refractories may be applied on the surfaces of afilament such as of tungsten or tantalum. It shall be noted that thepresent invention shall not be limited to coated lilaments, but thatnon-metallic filaments ofthe Nernst type having a preferred compositionof about percent thoria and about l5 percent yttrium oxide have alsobeen satisfactory. i i

The specific details of the apparatus and the essential characteristicsthereof for the preparation of high-purity uranium, plutonium, neptuniumand other metals are claimed in co-pending application of Theodore T.Magel, Serial No. 762,840, led July 23, 1947.

What is claimed is: y

l. The method of preparing uranium of extremely high purity from impureuranium consisting in passing a halogen continuously over the mass toform uranium halide vapor, and immediately passing the halide vapor intocontact with a non-metallic refractory surface having a temperatureabove the melting point of uranium and which is substantially unreactiveto the uranium above said melting point thereby decomposing the halide,splitting off elemental halogen and precipitating the uranium in moltenform.

2. The method of preparing uranium of extremely high purity whichconsists of heating a volatile halide of the uranium to form a halidevapor, contacting the heated halide vapor at sub-atmospheric pressurewith a refractory metal filament coated with a non-metallic refractoryselected from the group consisting of refractory metal oxides,refractory metal nitrides and carbon which is maintained at atemperature above the melting point of uranium and which issubstantially unreactive with the uranium above said melting point,thereby decomposing the halide and precipitating the uranium therefromon the filament in molten condition, and allowing the uranium to dripfrom the lament.

3. The process of producing uranium metal of extremely high purityconsisting in decomposing a vapor of a relatively volatile uraniumiodide by bringing the vapor in contact with a refractory metal filamenthaving a refractory metal oxide surface coating non-reactive with saidmetal in molten form and heated to a temperature above the melting pointof the uranium metal whereby elemental iodine is split off and .uraniumis deposited in pure molten form.

4. The method of preparing uranium of extremely high purity from animpure mass of uranium which consists of passing a halogen over animpure mass thereby forming a volatile halide of uranium, andimmediately contacting the halide vapor while maintaining it at lowpressure with a refractory metal lament having a rel fractory metaloxide surface coating and a temperature above the melting point ofuranium whereby elemental halogen is split off and uranium is obtainedin pure molten form, said refractory metal oxide being non-reactive withmolten uranium.

5. The method of preparing uranium of extremely high purity from crudeuranium metal with a preselected halogen consisting in continuouslypassing the halogen over an impure mass of the uranium at a temperatureto form uranium halide vapor, immediately passing the halide vapor overa refractory thorium dioxide coated tungsten filament having a surfacewhich is substantially unreactivc with the uranium at a temperatureabove the melting point of the uranium while maintaining the surface ofthe filament above said melting point, 'thereby 'D thermally decomposingthe halide and depositing the uranium thereon in molten condition.

6. A method of preparing uranium of extremely high purity consisting inheating iodide of uranium to a ternperature between 450 C. and 550 C. tovaporize the iodide and immediately contacting the iodide vapor with anon-metallic refractory surface having a temperature of from 1350 C. to1550 C. and which is substantially nonreactive with uranium within saidlatter range of temperature, maintaining the contact of the iodide andsurface sufficiently long to decompose the uranium iodide withdevelopment of elemental iodine and deposit uranium on the surface, andallowing the uranium to collect on the body in a molten state and todrip from the surface.

7. A method of preparing uranium of extremely high purity consisting inheating iodide of uranium to a temperature between 450 C. and 550 C. tovaporize the iodide and immediately contacting the iodide vapor with 5 arefractory body comprising a tungsten filament coated with thoriumoxide, said body having a temperature of from 1350 C. to 1550 C. andwhich is substantially nonreactive with uranium Within said latter rangeof ternperature, maintaining the contact of the iodide and filamentsufficiently long to decompose the uranium iodide and deposit uranium onthe filament, and allowing the uranium to collect lon the lbody in amolten state and to 'drip therefrom.

`8. Ay method 'of preparing uranium of Vextremely high purity consistingin heating iodide of uranium to a ternperature between '450 C.and 550 C.to vaporize the iodide and immediately ,contacting the iodide vapor witha refractory body comprising a refractory metal filament coated withtantalim nitride, said body having a temperature of from 1350, C. to1550 C. and which is substantially nonreactive with uranium within saidlatter range of temperatnre, maintaining the contact of the iodide andfilament 'sufficiently long to decompose the uranium iodide withldevelopment of elemental iodine and deposit uranium on the filament,and allowing the uranium to collect on'the body in a molten state and todrip therefrom.

9. A methodk of preparing uranium of extremely high purity consisting inheating iodide of uranium to a temperature between 450 C. and 550 C. tovaporize the iodide and immediately contacting 'the iodide vapor with arefractory body comprising a refractory metal filament coated withlanthanum oxide, said body having a ternperature of from 1350 C. to 1550C. and which is substantially nonreactive with uranium within saidlatter range of temperature, maintaining the contact of the iodide andfilament sufficiently long to decompose the uranium iodide with.development of elemental iodine and deposit uranium on the filament,and allowing the uranium to collect on 'the body in a molten state andto drip therefrom.

10. The process of purifying crude metallic uranium consisting inpassing iodine vapor over the crude uranium to be purified at atemperature to form uranium iodide in the vapor phase, and/thereaftercontacting the uranium iodide vapor with -a refractory body consistingof a refractory metallicfilament coated with arefractory metal oxidewhich is non-reactive with molten uranium 'and heated to a temperatureof from 1350 C. to 1500 C. which is sufficiently elevated to decomposethe uranium iodide whereby elemental iodine is split oi and puremetallic uranium is deposited on the refractory body in a moltencondition.

References Cited in the file of lthis patent UNITED STATES PATENTS OTHERREFERENCES Mellor: Inorganic and Theoretical Chemistry, vol. 12 (1932),publ. by Longmans, Green & Co., London, p. 93.

Alnutt et al.: Electrochemical Society, Preprint 88-30, Zirconium Metal,`Its Manufacture, lFabrication and Properties (vol. 88 of vtheTransactions),. October 17, 1945, 357-366.

1. THE METHOD OF PREPARING URANIUM OF EXTREMELY HIGH PURITY FROM IMPUREURANIUM CONSISTING IN PASSING A HALOGEN CONTINOUSLY OVER THE MASS TOFORM URANIUM HALIDE VAPOR, AND IMMEDIATELY PASSING THE HALIDE VAPOR INTOCONTACT WITH A NON-METALLIC REFRACTORY SURFACE HAVING A TEMPERATUREABOVE THE MELTING POINT OF URANIUM AND